1. Field of the Invention
The present invention relates to the field of energy conversion, particularly to conversion of radiant energy (light) into mechanical energy (movement).
2. Related Art
Presented below is background information on certain aspects of the present invention as they may relate to technical features referred to in the detailed description, but not necessarily described in detail. That is, individual parts or methods used in the present invention may be described in greater detail in the materials discussed below, which materials may provide further guidance to those skilled in the art for making or using certain aspects of the present invention as claimed. The discussion below should not be construed as an admission as to the relevance of the information to any claims herein or the prior art effect of the material described.
Sunlight provides a vast resource that has spurred the development of various methods to convert photons into work: photovoltaics for conversion to electricity, solar thermal for water heating, fast growing plants to produce bio-fuels, and solar water splitting to produce hydrogen and oxygen (Ref. 1). Though useful, these disparate methods are often based on complicated, capital intensive, multistage processes: (Ref. 2) light is collected and converted to a high energy intermediate (e.g., electrical potential, thermal loading, or chemical fuel), which is then used to run a process, such as an engine, that performs work. This multistage approach is ubiquitous and allows for a myriad of applications, but requires production, transportation, and possible storage of intermediates. Considerable effort has been devoted to improving energy collection, storage, and utilization (Ref. 2, 3); however, strategic simplification through the removal of the intermediates remains under-investigated and could provide reductions in capital costs.
Few strategies exist for the direct conversion of light into work. Concepts such as the solar sail (Ref. 4) for interstellar travel and optical trapping of small particles (Ref. 5) rely on weak momentum transfer from photons. Harnessing the energy of photons is a far more powerful process. A few opto-thermal methods have been developed (Ref. 6); for example, the Crookes Radiometer produces rotational motion in part via light-based heating of gases, but requires stringent conditions (i.e., low pressure) to function. When applied to liquids, local heating can produce thermally induced surface tension gradients. Such gradients have been shown to induce thermocapillary convective flows in oils (Ref. 7), and, as first realized by Brochard-Wyart et al. (Ref. 8), can move silicone oil droplets on hydrophobic surfaces under precisely defined conditions or water droplets in oil filled channels (Ref. 9). Nature has shown that surface tension gradients, created from chemical gradients, can move insects on the surface of water (Ref. 10). Similarly, camphor chips (Ref. 11), soap boats (Ref. 12), and decomposing hydrogen peroxide (Ref. 13) have been used to produce surface tension gradients and motion in the laboratory, but all inherently rely on the supply of exhaustible chemical intermediates to function.
The invention described below comprises methods that couple optical heating with the capability of surface tension gradients to move objects on the surface of liquids or at the interface between two liquids. With this approach, a direct means of converting light into useful work is realized, and a simple strategy for remotely powering and controlling small objects is demonstrated. A simple and robust solid/liquid interfacial system can convert light directly into useful work through thermal surface tension effects. This is demonstrated by the propulsion of objects on the surface of water. The simplicity of the system allows for controlled linear motion and rotational motion.